The present invention relates broadly to phase locked loop amplifiers, and in particular to a wide-band phase locked loop amplifier apparatus.
Phase locked-loop (PLL) systems typically include a voltage controlled oscillator (VCO) to produce the output signal. The operation of voltage controlled oscillators is typically non-linear in that f=KV where f is the output frequency, K is a constant and V is the input voltage. Where the desired output frequency changes in some predetermined manner as a function of time, it is known to incorporate a memory which stores voltage signals. The stored signals provide the appropriate voltage input to the voltage controlled oscillator as a function of time and include the correcting voltages to compensate for the voltage controlled oscillator non-linearities.
The memory, which is usually a digital memory, is combined with a digital-to-analog (D/A) converter to produce voltage signals as a function of time to be added to the conventional phase error signal. The resulting composite signal, when applied to the voltage controlled oscillator input, is desired to be of correct shape (vs. time) as to yield minimum phase error in the voltage controlled oscillator output. Such a modified system in the present art may fail to reduce the errors to the minimum since linearity errors in the voltage controlled oscillator change with temperature, long-range time and with changes in phase locked loop components.
The phase lock loops which are utilized in current stabilized local oscillators (Stalos) are operated as tracking filters. These tracking filters are utilized to suppress both reference noise which occurs outside the loop bandwidth and the voltage controlled oscillator (VCO) noise which occurs inside the loop bandwidth. Since the reference signal is crystal generated, the reference noise is orders of magnitude better than the voltage controlled oscillator (VCO) noise making wide-band phase lock loops desirable. The added advantage of using wide-band loops is the faster acquisition and lock-up times. In the past, wide-band loops were difficult to achieve because of the phase delay which was introduced by the loop amplifier and loops greater than two MHz were unstable.
The present wide-band phase locked loop amplifier apparatus solves the problem of the added phase delay in the loop amplifier by using two frequency paths. By using this technique, the active loop amplifier is able to achieve gain-bandwidth products of 10 to 20% of the loop bandwidth. In the present invention, the active amplifier gain-bandwidth product was 5 to 10 times the loop bandwidth to ensure an acceptable phase margin; a reduction by a factor of 50 in the requirement of the active amplifier. The existing loops could be easily altered whenever unity gain loop amplifiers are used, to minimize phase delay, and to make a more stable loop.